If CoC scratches that itch for me, why would I want either of the other two games?
I mean, my feeling is usually kind of the reverse, maybe I'm a neophiliac or whatever but if a game is like, absolutely in my "strike zone" interests-wise, i.e. it's very similar to a game I like in terms of subject matter, then that automatically makes me
more interested in it, not less. What would make me less interested isn't "It's about that subject you like running RPGs about!", but rather "The rules are rather similar to that RPG about that subject that you already have!" or "This game just applies generic-ass rules to the subject". Hence yet another d20-based, level-based, linear-power-gain Heroic Fantasy RPG doesn't necessarily make me go "Ooooh!", but a gain using very different mechanical approaches and ideas to explore Heroic Fantasy or a similar area does make me pay attention.
I don't think this is uncommon either - like, there's a whole bunch of Cthulhu mythos games for a reason, and I don't think it's just a lot of people thinking "CoC sux lol" or something, but rather because people like seeing different ways it could be done, and like trying new games with similar subject matter to see if they prefer them.
And trying is part of it because like, how does one know that, say Arkham Horror and Candela Obscura won't do a better job than CoC? I'm not saying they will, but I'm making the point that most people don't just
know that - that wanting to find that out by playing them or at least reading them is actually very common.
if I've already got Mage, then what do I need WitchCraft for?
I guess if you want to run something more like The Craft or a darker take on Buffy rather than The Matrix/Jujutsu Kaisen? I mean, those two games seem like they're about very different genres to me.